How exactly do I get over a guy and get him back for this... after dating for 4 YEARS ?

me and this guy dated for 4 years then we had a mutual break-up and we were fine. But then he started to go out the following week with this girl ( that I hate which also hates me back ) they keep rubbing it in face that they're together... at first it didn't bother me but he started playing me because i'm not over him yet! we went out as friends and out of nowhere HE KISSED ME and we started talking about getting back together... the following week he stoped acting cute with me and went back to her I don't know how to get over him and how I can get him back ! HELP PLEASE